1. Field of the Invention
This invention relates to a process for producing toner particles used in image forming methods in which electrostatic latent images are rendered visible, or in toner jet recording methods, and a toner having such toner particles.
2. Related Background Art
A number of methods are conventionally known as electrophotography. In general, copied images are obtained by forming an electrostatic latent image on an electrostatically charged image bearing member (hereinafter also “photosensitive member”) by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a visible image (toner image), transferring the toner image to a transfer medium such as paper, and then fixing the toner image onto the transfer medium by the action of heat and/or pressure.
As for printers, LED printers or LBP printers are prevailing in the recent market, and are required to have higher resolution sop that the resolution has been raised from 240 or 300 dpi to 400, 600 or 800 dpi. With such resolution, higher definition has come to be required. Copying machines are developed to have advanced functions, resulting in progress toward digitalization. Since digitalization is primarily involved in a method in which electrostatic latent images are formed by using laser light, the progress also is directed to higher resolution, so that as with printers, developing methods are required to higher resolution and higher definition. As one means for satisfying this requirement, toner particle diameters have been progressively reduced, and a toner has been proposed having toner particles with small particle diameters in specific particle size distribution (see, e.g., Japanese Patent Application Laid-open No. H09-179332)
In recent years, there have been a tendency toward decreasing toner particle diameters in order to achieve higher resolution and higher definition, and the smaller the toner particle diameters, the more important the stable triboelectric charging of the toner. More specifically, unless each of the fine toner particles has been equally charged, image stability is apt to be remarkably lowered. The reason therefor is considered to be that toner particle diameters are simply reduced, and as compared with the coulomb force applied in a transfer step, the adhesive force of the toner to a photosensitive member (mirror image force or van der Waals force) comes to be too large, and as a result, residual toner increases, and besides, since reduction in toner particle diameters is accompanied by deterioration in flowablity, each toner is liable to be unevenly charged, toner particles causing fogging or inferior transfer increase.
In order to improve the performance of toner, it is essential for the toner to retain stabler charge characteristics. Factors to determine the charge characteristics of toner are roughly classified into the quantity of electric charges produced by friction between toner particles and the quantity of electric charges produced by friction or contact between toner particles and external members, where the charge characteristics of toner are greatly concerned with the surface material, and the size and shape of toner particles and the distribution states thereof, of each toner particle, and the influence of external additives aiming at auxiliary charging, control members making use of a metallic or rubber material, and charge control agents which are components included in toner particles.
For example, in the production of electrostatic latent image developing toners which produces toners by suspension polymerization, a method is proposed which enables particle shapes to be controlled and produces toner particles having small particle diameters and sharp particle size distribution, i.e, particle size distribution concentrated in a narrow range (see, e.g., Japanese Patent Application Laid-open No. H10-312086). This proposal is characterized in that a certain speed gradient and pH range are applied in machine agitation at the time of preparing an aqueous dispersion medium, and further, high-speed rotating shearing agitation having a certain speed gradient is carried out also at the time of granulating polymerizable monomers.
In respect of a method of preparing a sparingly water-soluble inorganic salt in an aqueous dispersion medium in suspension polymerization, it is proposed that pH is precisely controlled at the time of preparing the inorganic salt, the resulting suspension polymerization toner is reduced in its particle diameter, and particle size distribution is concentrated in a narrow range (see, e.g., Japanese Patent Application Laid-open No. H07-49586).
In all the above proposals, a certain effect can be obtained by concentrating the particle size distribution in a narrow range, but the effect is insufficient especially in respect of the control of fine particles having smaller particle diameters than the intended small-diameter toner particles. In the case where toner particles including such fine particles are used, even if the developing performance is satisfactory at the initial stage, it would be unsatisfactory in respect of transfer performance and anti-fogging properties, as continuous printing is repeatedly carried out in various environments. It is further disclosed that, in a method of producing toner particles by dispersing a polymerizable composition in an aqueous medium, an alcohol is added to the aqueous medium, (see, e.g., Japanese Patent Application Laid-open No. H05-197185). In such a proposal, however, the concentration of the alcohol is not controlled throughout polymerization reaction, and hence, even if the resulting toner has sufficient durability in a normal temperature and normal humidity environment, problems are apt to be raised in transfer performance and anti-fogging properties due to environmental variations. Thus, room is still left for improvement, and a toner is desired to satisfy the above requirements.